Producing materials having specific required properties is steadily gaining in importance and, hence, the field of combinatorial chemistry, which generally refers to methods for creating collections of diverse materials or compounds, commonly known as libraries, is steadily increasing and has revolutionized the process of drug discovery. Combinatorial chemistry enables researchers to rapidly discover and optimize useful materials such as polymers, superconductors, magnetic materials, etc. In order to record the various properties of the materials obtained by combinatorial chemistry, it is necessary to precisely and efficiently determine the characteristics of the materials, preferably under varying environmental conditions. One important and useful characteristic is the behavior of a material, in particular of a polymer, when exposed to an eternally-applied shear force. Instruments for measuring the response of fluids to applied shear are generally referred to as Theological instruments, which may be subdivided into indexers and rheometers. Indexers measure a quantity which is correlated with the rheological characteristics, but which is difficult to analyze in terms of intrinsic material properties. Although indexers can be assembled rapidly from commercially available components, the results of measurements carried out by means of such indexers are difficult to relate to results of measurements from other indexers, or to intrinsic material properties, without extensive calibration.
Rheometers, on the other hand, measure intrinsic material characteristics, giving them broad applicability. Such generality, however, comes at a price due to design costs and complexity which are dictated primarily by the need for well-defined static and dynamic test conditions. Moreover, these known rheometers require fairly large quantities of sample in the order of 500 mg so as to obtain the required accuracy in analyzing the samples. Such large quantity samples, however, are usually not provided by combinatorial synthesizing methods in which, generally, a large amount of differing samples of small quantity are produced.
Rheological measurements on sample materials, such as polymeric materials, are performed in their simplest geometry such that the sample is placed between two parallel plates of a design area separated by a gap of known distance, wherein the sample is sheared by applying a force to one of the plates while keeping the other plate fixed. This results in a displacement, i.e. a deformation of the sample confined between the plates, which can be characterized in terms of the shear stress and the shear strain. From these quantities and the dimensions of the sample, a shear modulus may be calculated. In general, the shear modulus is a function of the sample history, the shear strain and the strain rate. For polymeric materials, the temporal dependence of the shear modulus at constant stress typically exhibits four different regimes reflecting different relaxation mechanisms available to the polymer chain.
For sufficiently small deformations, most polymers exhibit linear viscoelastic behavior in which the shear modulus is independent of the shear strain. Theories of polymer dynamics generally explain the response of a chain in terms of normal modes, each having a characteristic frequency. The linear viscoelastic theory gives, then, the response of the material as a function of shear history. In measuring the mechanical property of a sample material, the sample is subjected to a varying force, e.g. a sinusoidal-varying force, and the resulting deformation, i.e. the response of the sample is observed. The frequency response of the sample may then be analyzed in accordance with viscoelastic theories to obtain information on the required characteristic of the material.
In order to perform these measurements with a high degree of accuracy, the rheological apparatus must be capable of producing a well-defined displacement within a specified frequency range. Additionally, since each frequency corresponds to probing the response of the sample material at a particular relaxation time, such measurements take a relatively long time period when the relaxation mechanism of the samples requires the employment of low frequencies. Hence, measuring a plurality of samples which may be produced by combinatorial chemistry is a very time-consuming and therefore very expensive procedure.
Moreover, the performance of accurate measurements requires the application of suitable sensor elements for detecting the shear stress in the samples. In order to obtain meaningful experimental results, the sensor elements have to be suitably designed so as to reflect the response of the sample to the applied shear strain without any interference or at least minor interference of the sensor element.
In view of the above-mentioned problems, it is an object of the present invention to provide apparatus and methods for rheological measurements which are capable of producing reliable measurement results for a plurality of small quantity samples within a short time period.